API RP 534:2007 pdf download

API RP 534:2007 pdf download.Heat Recovery Steam Generators.
2.3.1.4 Pinch Temperature
The degree to which the heating medium is required to approach the steam saturation temperature strongly affects the HRSG As the design pinch temperature is reduced, the log mean temperature difference (LMTD) decreases and the surface area requirement increases. HRSGs with large pinch temperatures tend to use larger diameter or shorter tubes than those with small pinch temperatures. The typical pinch temperature range is 8°C — 14°C (15°F — 25°F).
2.3.1.5 Outlet Temperature Control
Certain processing applications require close control of the heating medium outlet temperature. For instance, secondary reformer effluent in an ammonia plant enters a CO to CO2 shift reactor after being cooled by the firetube HRSG. Overcooling by the HRSG adversely affects the shift reaction catalyst. For this reason, such firetube HRSGs incorporate a hot gas bypass system, which may be either internal or external to the HRSG Refer to 2.6.1.13 for further construction details.
The amount of gas bypassed is a function of turndown, extent of fouling, and the design temperature approach. The equipment tends to overcool the heating medium when run at reduced throughput and when clean. HRSGs with large design approaches tend to overcool due to the large approach (serving as thermal driving force) at the outlet end. Such units require large bypass systems for temperature control.
2.3.1.6 Gas Dew Point
Process fluid gas streams which may reach the dew point of one of the gas constituents require special attention. Condensation can occur on cold surfaces such as the tubes and refractory lined walls even though the bulk gas temperature may be above the dew point. If bulk gas cooling below the dew point occurs, as in sulfur recovery boilers, provision must be made to ensure condensate removal. For acid gases, material selection is more important with respect to dew point than condensate removal.
2.3.2 Boiler Feed Water/Steam
Appendices A and B provide general information with regard to the boiler feed water/steam system. Additional considerations unique to firetube equipment are covered in 2.3.2.1 and 2.3.2.2.
2.3.2.1 Heat Flux
Maximum allowable heat flux rates for firetube HRSGs are a function of equipment construction details, steam pressure, recirculation rates, water quality, etc. Specific construction features which affect flux limits include:
a. Tube quantity. diameter and pitch: in general, flux limits are lower for increasing tube quantity or decreasing pitch to diameter
ratio.
b. Quantity, size, and location of risers and downcomers.
c. Clearance between bundle and shell.
Actual flux rates for comparison with design limits are based on clean tube surface at the tube inlet where the process fluid is the hottest. Firetube HRSG design should account for increased hot process fluid heat transfer coefficients due to tube entrance effects.
2.3.2.2 Boiler Water Circulation
Critical service, high-temperature firetube HRSGs are furnished with elevated steam drums, from which boiler water is supplied with high circulation rates. Systems may be either natural or forced circulation, with the former being most common.
Low-flux HRSGs may also be furnished with an external drum. However, such HRSG equipment more commonly makes use of an expanded shell-side compartment with the tube bundle submerged in the boiler water. Liquid disengagement occurs above the established liquid level within the expanded shell. Such a unit is commonly referred to as a kelly type boiler. Natural circulation patterns occur within the kettle shell. A water-steam mixture rises through the tube bundle: the vapor rises through the steam! water interface to the steam space above; and the boiler water recirculates back down each side of the bundle to the bottom of the shell. The kettle HRSG shell serves the purposes of a steam drum in a conventional boiler system. It differs from a conventional drum in that the HRSG heating surface is self contained, connections are altered, and steam/water internal flow patterns are different. Saturated steam generated in kettle HRSGs is normally used for non-critical services so that the requirements for purity and quality (see Appendix A) may be relaxed. Therefore, separation is commonly achieved by deflector plates or dry pipes. See 2.6.2.4 for additional shell details.